C Program?

Question

A program that implement string matching in detecting plagiarism in two documents.

Answer 1

Look up the algorithm that finds the "longest common subsequence". Modify it as needed to fit your requirement.
For example, you could invent a ranking score for plagiarism based on a weighted combination that includes the longest subsequence length, the number of subsequences that are in common, and the overall percentage of common subsequence lengths over the total number of characters.
EDIT: I did a little legwork for you. Here is a source where you can learn two techniques of finding the longest common subsequence:
https://www.ics.uci.edu/~dan/pubs/p664-hirschberg.pdf